AbstractÐStereochemically constrained amino acid residues that strongly favour speci®c backbone conformations may be used to nucleate and stabilize speci®c secondary structures in designed peptides. An overview of the use of aa-dialkyl amino acids in stabilizing helical structures in synthetic peptides is presented, with an emphasis on work carried out in the authors laboratory. a-Aminoisobutyric acid (Aib) and related achiral homologs facilitate stable helix formation in oligopeptides as exempli®ed by a large number of crystal structure determinations in the solid state. The ability to design conformationally rigid helical modules has been exploited in attempts to design structurally well characterized helix-linker±helix, using potential nonhelical linking segments. b-Hairpin design has been approached by exploiting the tendency of `prime turns ' to nucleate hairpin formation. The use of nucleating D Pro-Gly segments has resulted in the generation of several well characterized b-hairpin structures, including the crystallographic observation of b-hairpin in a synthetic apolar octapeptide. Extensions of this approach to three stranded b-sheets and larger structures containing multiple D Pro-Gly segments appear readily possible
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